row_number() from dplyr: An example with vector

Question

I don't understand this example from the dplyr help for row_numbr() using ?row_number().

library(dplyr)
x <- c(5, 1, 3, 2, 2, NA)
row_number(x)
[1]  5  1  4  2  3 NA

Why is the result not c(1, 2, 3, 4, 5, 6)? With mtcars:

mtcars %>% 
  select(mpg, cyl) %>% 
  mutate(row_num = row_number()) %>% 
  head()

which just gives the row number:

   mpg cyl row_num
1 21.0   6       1
2 21.0   6       2
3 22.8   4       3
4 21.4   6       4
5 18.7   8       5
6 18.1   6       6

Answer 1

If you look at help("row_number"), then you can find that:

row_number(): equivalent to rank(ties.method = "first")

Moreover, as you vector contains also NA, it should be noted that:

x (is) a vector of values to rank. Missing values are left as is. If you want to treat them as the smallest or largest values, replace with Inf or -Inf before ranking.

What can be used instead is:

seq_along(x)

[1] 1 2 3 4 5 6